Communication apparatus and communication system

ABSTRACT

A communication system including two layers, each having a switching function. The two layers includes lower-layer apparatuses which are connected with communication lines of a lower layer and communication lines of an upper layer and upper-layer apparatuses which are connected with the communication lines of the upper layer. Line failure in the lower and upper communication lines is detected and line switching is coordinated by the line-switching function of the lower and upper-layer apparatuses by using failure information on the lower and upper-layer communication lines.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a communication apparatus and acommunication system.

2. Description of the Related Art

Data communication has recently become more and more important with thespreading Internet, etc. as its background. ATM networks in accordancewith ITU-T 1.150 have been introduced in this field to integratetelephony and data communication. High-speed optical communicationnetwork technologies such as SONET defined by ANSI T1.105, SDH definedby ITU-T G.707, and OTN defined by ITU-T G.872 have been introduced inthis field. SONET and SDH have basically the same function, and OTN, anoptical network, is based on the WDM (Wavelength Division Multiplexing)technology to raise the transmission capacity in a fiber.

These networks are interconnected by prescribing upper and lower layersbased on the layer hierarchy of the OSI (Open System Interconnection)reference model defined by ISO. According to the provisions of theseindividual networks, ATM is ranked at the top; OTN, at the bottom; SONETand SDH, at the middle. Each network is also of hierarchic structure,consisting of subnetworks. For example, an ATM network consists of avirtual path and a virtual channel, and SONET/SDH and OTN are ofhierarchic structures, consisting of subnetworks, as well.

SUMMARY OF THE INVENTION

Table 1 shows the relations between such upper and lower layers asrelate to switching in the above subnetworks, segments defined byrepeaters excluded.

FIG. 1 shows the relation between defined segments and multiplexing in alower and an upper layer. Numerals 100-102 and 110-112 indicateapparatuses in the upper layer; 300 and 310, apparatuses in the lowerlayer; 500-502, communication lines in the upper layer; 600, acommunication line in the lower layer.

In FIG. 1, a plurality of upper-layer apparatuses is connected to alower-layer apparatus. The upper-layer apparatuses 100-102 are connectedto the lower-layer apparatus 300 through the upper-layer communicationlines 500-502. The upper-layer apparatuses 110-112 are connected to thelower-layer apparatus 310 through the upper-layer communication lines500-502. The lower-layer apparatuses bundle the upper-layercommunication lines by TDM (Time Division Multiplexing) of SONET of ANSIT1.105 or SDH of ITU-T G.707 or by WDM of OTN of ITU-T G.872 so as toraise the data transmission capacity per physical medium or unit timeand transmit data through the lower-layer communication line.

The upper-layer apparatuses 100 and 110 process the overhead portions ofSONET or SDH signals when they transmit and receive the signals throughthe upper-layer communication line 500, the overhead portions containingthe administration control signals of the upper-layer communicationline. The upper-layer apparatuses 101 and 111, and 102 and 112 carry outthe same processing for the upper-layer communication lines 501 and 502,respectively. The segment for processing the administration controlsignals of the upper-layer communication line is called upper-layercommunication-line segment (hereinafter “upper-layer segment”). Theupper-layer communication lines 500-502 may be three physically separatelines or such a physically single line comprising three separate logicallines as the bus of SONET or SDH. In the same way, the lower-layercommunication line 600 is terminated by the lower-layer apparatuses 300and 310, and the segment between the apparatuses is called thelower-layer communication-line segment (hereinafter “lower-layersegment”).

In this way, signals of the upper layer are multiplexed to becomesignals of the lower layers.

Accordingly, the signals of the upper layer may become equal to butnever become larger than the signals of the lower layer in terms of thesignal band. The defined segments of the upper layer may be equal to butnever shorter than the defined segment of the lower layer.

FIG. 2 shows a case wherein each of the upper and lower layers hasprotection-switching processing parts. Upper-layer communication lines500 to 505 are terminated by upper-layer apparatuses 100-104 and signalprocessing parts 120-124 and 125-129 in the apparatuses 100-104.Multiplexing/demultiplexing parts 440 and 450 in lower-layer apparatuses300 and 310 multiplex and demultiplex the upper-layer communicationlines 500-503, which, going through the lower-layer segment of thelower-layer communication line 600 or 601, connect the upper-layerapparatuses 100-103 with those 110-113. In the same way,multiplexing/demultiplexing units 441 and 451 in lower-layer apparatuses301 and 311 multiplex and demultiplex the upper-layer communicationlines 504-505, which, going through the lower-layer segment of thelower-layer communication line 602 or 603, connect the upper-layerapparatuses 103 and 104 with those 113 and 114.

For the sake of simplification of description, a protection switchingsystem of the 1+1 type will be taken as an example and its descriptionwill follow. The system comprises a set of a working communication lineand a protection line. While the system is operating normally, the samedata are transmitted through both the working and protection lines andthe receiving side chooses a line of which the transmission quality isbetter than that of the other. In FIG. 2, a lower-layercommunication-line bridge/selector part 420 or 430 on the transmittingside transmits the same data through both the working and protectionlower-layer communication lines 600 and 601. In the same way, alower-layer communication-line bridge/selector part 421 or 431 on thetransmitting side transmits the same data through both the working andprotection lines 602 and 603. While all the communication lines and allthe apparatuses are normal, the lower-layer communication-linebridge/selector part 430 or 420 on the receiving side chooses theworking line 600. When the working line 600 goes out of service due tosome failure, the lower-layer communication-line bridge/selector part430 or 420 on the receiving side chooses the protection line 601 torestore the communication line in the lower layer. In the same way,while all the communication lines and all the apparatuses are normal,the lower-layer communication-line bridge/selector part 431 or 421 onthe receiving side chooses the working line 602. When the working line602 goes out of service due to some failure, the lower-layercommunication-line bridge/selector part 431 or 421 on the receiving sidechooses the protection line 603 to restore the communication line in thelower layer.

As in the case of the lower-layer communication lines, an upper-layercommunication-line bridge/selector part 220 or 230 on the transmissionside transmits the same data through both the working and protectionupper-layer communication lines 503 and 504. While all the lines and allthe apparatuses are normal, the upper-layer communication-linebridge/selector part 230 or 220 on the receiving side chooses theworking line 503. When the working line 503 goes out of service, theupper-layer communication-line bridge/selector part 230 or 220 on thereceiving side chooses the protection line 504 to restore thecommunication line.

Describe below is the switching processing in the upper and lower layersupon the occurrence of failure in the lower-layer communication line 600as shown in FIG. 3. The switching method was devised to describe theembodiments of the present invention. FIG. 3 shows a portion includingthe protection-switching processing parts of FIG. 2. In this method,when failure has occurred in the lower layer, an alarm indication signal(AIS) is sent to the upper-layer apparatuses. FIG. 4 shows the conceptof AIS. As shown in FIG. 4, upper-layer communication lines 500-502 areterminated by upper-layer apparatuses 100-102 and 110-112. Lower-layerapparatuses 300 and 310 connect the upper and lower layers, and thelower-layer apparatuses 300 and 310 are connected to each other by alower-layer communication line 600. Now it is assumed that failure hasoccurred in the lower-layer communication line 600. In FIG. 4, signalstransmitted in both directions of the lower-layer communication line 600are shown. Now a case wherein the failure has effects in both directionsis considered. At first, each of the lower-layer apparatuses 300 and 310detects the failure in the lower-layer communication line 600 and sendsout an AIS through all the upper-layer communication lines 500-502.Accordingly, all the upper-layer apparatuses 100-102 and 110-112connected to the lower-layer apparatuses recognize that failure occurredin the lower-layer communication line 600 and the data transmittedthrough the upper-layer communication lines 500-502 are invalid. The AISis prescribed in each layer of each network shown in Table 1; i.e., thevirtual path and the virtual channel of the ATM network, the path andthe line of the SONET, the path and the M section of the SDH network,the optical channel and the optical multiple section of the OTN network,and so on. The provisions of each network prescribe the AIS from thelower layer to the upper layer in each network in accordance withTable 1. Internetwork AIS is also prescribed. For instance, ITU-T 1.610prescribes the AIS for the case wherein the lower layer is a SONET andthe upper layer is an ATM network.

FIG. 5 is a conceptual illustration of the AIS from a lower layer to anupper layer. The line connecting the upper layer and the lower layer isthe upper-layer communication line, and the line connecting the twolower-layer apparatuses of the lower layer is the lower-layercommunication line. AIS is made from the lower layer to the upper layerbut not from the upper layer to the lower layer.

With this AIS system, the upper layer can recognize the failure whichhas occurred in the lower layer. In this method, theprotection-switching processing part of each of the upper and lowerlayers have independent protocol and determines independently whether toswitch the communication line or not based on failure informationdetected or an AIS received, as the case may be. When failure hasoccurred in the-lower-layer communication line 600 of FIG. 3, thelower-layer apparatuses 300 and 310 detect the failure and begin theswitching processing to switch the communication line from the workingline 600 to the protection line 601 in the lower layer. On the otherhand, the upper-layer apparatuses 100 and 110 begin the switchingprocessing upon their receipt of an AIS to switch the communication linefrom the working line 503 to the protection line 504 in the upper layer.In this way, line switching takes place in both the lower and upperlayers even when line switching is required in only one layer for therestoration of a failed line. To prevent such unnecessary lineswitching, either the lower-layer switching function or the upper-layerswitching function may be disenabled permanently. For example, ANSIT1.105 prescribes NUT (Non-preemptible Unprotected Traffic) to prohibitline switching path by path of SONET apparatuses, assuming that a SONETapparatus is connected to a network with a switching function such as anATM network, the former constituting the lower layer and the latterconstituting the upper layer.

Another shortcoming anticipated with the system wherein switching ismade in both the upper and lower layers is that while the lower-layerapparatuses are switching the communication line, the upper-layerapparatuses may not correctly send and receive the switching protocolbetween them. One method of coping with this problem would be to stopthe switching processing in either of the two layers for a prescribedtime period. For instance, ITU-T I.630 prescribes the hold-off time forthe system consisting of an ATM network and a SONET or SDH network.While the SONET or SDH network constituting the lower layer is switchingthe communication line, the switching processing of the ATM networkconstituting the upper layer is stopped for the hold-off time, of whichthe range and increment are 0-10 seconds and 500 msec.

As described above, a loss occurs in the switching time because theswitching in one of the two layers has to be stopped permanently or fora prescribed time period in case of a system consisting of a lower layerand an upper layer, each having a line-switching function. In the caseof the NUT method of ANSI T1.105, the protection-switching processingparts of a SONET become useless because the switching in the network isstopped. On the other hand, according to the method of ITU-T I.630,there occurs a waiting time of at least 500 msec in case that failurewhich cannot be dealt with in a SONET is to be dealt with in an ATMnetwork. FIG. 7 is the time chart of this method. When failure occurs inthe lower layer, the lower-layer apparatuses detect it and send out anAIS through all the upper-layer communication lines. Accordingly, theupper-layer apparatuses connected with the lower-layer apparatusesrecognize that failure occurred in the lower-layer communication lineand the data transmitted through the upper-layer communication lines areinvalid. However, because a hold-off time is set in the upper-layerapparatuses to prevent unnecessary switching as described above, theupper-layer apparatuses do not begin the switching processing during thehold-off time. In FIG. 7, it is assumed that the upper layer is an-ATMnetwork, the lower layer is a SONET or SDH network, and the hold-offtime is 500 msec.

The lower-layer apparatuses send out the AIS to the upper layer and, atthe same time, begin to switch the communication line. Because theprotection switching system is of a 1+1 type, the receiving apparatusesthat detected the failure in the communication line have theirbridge/selector parts switch the communication line from the workingline to the protection line. In the case of SONET, it is prescribed thatthe time necessary for the line switching should be 50 msec or less. Ifthe protection line also has trouble or the bridge/selector apparatusesdo not function correctly due to trouble of an apparatus, the lineswitching in the lower layer cannot be made. Namely, the lower-layerreceiving apparatuses keep detecting the failure in the lower-layercommunication line and sending out the AIS through the upper-layercommunication lines. But, the upper-layer apparatuses continue standingby until the hold-off time passes. Then, the upper-layer apparatusescarry out the switching processing. Namely, the upper-layer receivingapparatuses have their bridge/selector parts switch the communicationline from the working line to the protection line in the upper layer. Inthe case of ATM, the target value of the switching time of 50 msec isprescribed in ITU-T Draft New Recommendation I.630. After the lineswitching has been made successfully, the upper-layer apparatuses canreceive data through the protection line without detecting failure orreceiving an AIS. As shown in FIG. 7, SONET switches the communicationline within 50 msec. However, the difference of 450 msec between thehold-off time of 500 msec and the SONET switching time of 50 msec arewasted because action is taken in neither the upper nor the lower layer.

The object of the embodiments of the present invention is to achieveinterlayer-coordinated communication-line switching in a communicationsystem or network comprising two layers, each having a switchingfunction. In the interlayer-coordinated communication-line switching,the features of the two switching functions are utilized according tothe line conditions so that the interlayer-coordinatedcommunication-line switching can be made in a shorter time and is moreefficient and reliable than the switching in accordance with the priorart.

An embodiment of the present invention is as follows, and part of theconfiguration of the embodiment is shown in FIG. 21. A communicationsystem wherein (1) provided are a first and a second upper-layerapparatus and a first, a second, a third, and a fourth lower-layerapparatus, each apparatus having line-switching means, (2) (i) the firstand second upper-layer apparatuses are connected to each other through afirst and a second upper-layer communication line between whichcommunication can be switched, (ii) the first and second lower-layerapparatuses are connected to each other through a first and a secondlower-layer communication line between which communication can beswitched, each lower-layer communication line being multiplexed toaccommodate one or more upper-layer communication lines, (iii) the thirdand fourth lower-layer apparatuses are connected to each other through athird and a fourth lower-layer communication line between whichcommunication can be switched, each lower-layer communication line beingmultiplexed to accommodate one or more upper-layer communication lines,(iv) the first upper-layer communication line connects the firstupper-layer apparatus with the first lower-layer apparatus and thesecond upper-layer apparatus with the second lower-layer apparatus, and(v) the second upper-layer communication line connects the firstupper-layer apparatus with the third lower-layer apparatus and thesecond upper-layer apparatus with the fourth lower-layer apparatus, (3)a switching-inhibit notification is transmitted to the upper-layerapparatuses when the lower-layer apparatuses have detected line failure,(4) one or more faulty lines of the lower or the upper layer areidentified and the site of occurrence of failure is located by usingfailure information on the lower- and upper-layer communication lines,(5) the switching-inhibit signal to the upper-layer apparatuses iscancelled when no lower-layer communication line has been found faultybut any of the upper-layer communication lines has been found faulty,(6) which line-switching means of the lower- and upper-layer apparatusesshould make switching is determined, based on failure information on thelower- and upper-layer communication lines, in order to secure a largestnumber of normal upper-layer communication lines, or in order to restorehigh-priority lines rather than low-priority lines, or in order tosecure a largest number of signal channels, in case that any of thelower-layer communication lines has been found faulty, and (7) providedis a first means for causing line switching in the lower layer first andthen canceling the switching-inhibit signal to the upper-layerapparatuses, in case that switching is to take place in both the lowerand upper layers in accordance with said determination.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration for describing the embodiments of the presentinvention, and the concept of a lower-layer communication-line segmentand upper-layer communication-line segments is shown;

FIG. 2 is a block diagram for describing the embodiments of the presentinvention, and shown in this figure is the configuration of a networkcomprising two layers, each having switching parts;

FIG. 3 is a block diagram for describing the embodiments of the presentinvention and particularly for describing the switching processing inthe lower and upper layers in the case of occurrence of failure in alower-layer communication-line segment;

FIG. 4 is a block diagram for describing an example for comparison withthe embodiments of the present invention and particularly for describingthe transmission of an AIS from the lower layer to the upper layer;

FIG. 5 is a schematic block diagram for describing an example forcomparison with the embodiments of the present invention andparticularly for describing the concept of an AIS system between thelower and upper layers;

FIG. 6 is a block diagram for describing an example for comparison withthe embodiments of the present invention, and the configuration of theswitching apparatuses in the lower and upper layers of the example isshown;

FIG. 7 is an illustration for describing an example for comparison withthe embodiments of the present invention, and shown in this figure is atime chart of the switching in the lower and upper layers of theexample;

FIG. 8 is a schematic block diagram for describing an embodiment of thepresent invention, and shown in this figure is the concept of switchingprocessing in the lower and upper layers;

FIG. 9 is a block diagram for describing the embodiment of FIG. 8 andshows the configuration of the switching system with acoordinated-switching decision part;

FIG. 10 is a schematic block diagram for describing another embodimentof the present invention, and shown in this figure is the concept ofswitching processing in the lower and upper layers;

FIG. 11 is a block diagram for describing another embodiment of thepresent invention, and shows the configuration of the switching systemwith a coordinated-switching decision part;

FIG. 12 is a schematic block diagram for describing still anotherembodiment of the present invention, and shown in this figure is theconcept of switching processing in the lower and upper layers;

FIG. 13 is a block diagram for describing the embodiment of FIG. 12 andshows the configuration of the switching system with acoordinated-switching decision part;

FIG. 14 is a block diagram for describing a fourth embodiment of thepresent invention and shows the configuration of the switching systemwith a coordinated-switching decision part;

FIG. 15 is an illustration for describing an embodiment of the presentinvention, and shown in this figure is a time chart of the switching inthe lower and upper layers;

FIG. 16 is a flowchart of the processing carried out by thecoordinated-switching decision part of the first embodiment of thepresent invention of FIG. 8;

FIG. 17 is a functional block diagram of the coordinated-switchingdecision part of the first embodiment of the present invention of FIG.8;

FIGS. 18, 19, and 20 show examples of the processing, upon theoccurrence of failure, of the coordinated-switching decision part of thefirst embodiment of the present invention of FIG. 8; and

FIG. 21 is an illustration for describing an embodiment of the presentinvention, and shows the configuration of a network.

DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment

First described is a switching system having a coordinated-switchingdecision part in accordance with the first embodiment of the presentinvention. FIG. 8 is a conceptual illustration of the switching device.

In this embodiment, a decision part for coordinated switching isprovided. The decision parts receive failure information from an upperand a lower layer and determine in which layer to make switching inaccordance with failure conditions. Besides, the decision parts areconstructed so as to receive information on whether each upper-layerapparatus has a bypass, namely, another route going round thelower-layer apparatuses which are about to make switching. The lowerlayer sends the upper layer a switching-inhibit signal in addition to analarm indication signal (AIS). By sending out the switching-inhibitsignal to the upper layer, the switching in the lower layer can be madebefore that in the upper layer.

Before describing the switching system according to the first embodimentof the present invention, a comparative example will be described withreference to FIG. 5. FIG. 6 shows the details of a part of the networkof FIG. 2 in case that the network is constructed of conventional lower-and upper-layer apparatuses.

In FIG. 6, a lower-layer apparatus 310 is connected with anotherlower-layer apparatus through a working lower-layer communication line600 and a protection lower-layer communication line 601. The lower-layerapparatus 310 is also connected with upper-layer apparatuses 110-113through upper-layer communication lines 500-503. The upper-layerapparatus 113 has a protection upper-layer communication line 504 and isconnected with another lower- or upper-layer apparatus through theprotection line 504. The same data as the working upper-layercommunication line 503 can be transmitted through the protectionupper-layer communication line 504 as the occasion demands.

Now, the system will be described, assuming that signals are flowingthrough the lower-layer communication lines 600 and 601 and theupper-layer communication lines 500-504 from the left-hand side to theright-hand side in FIG. 6. The system with signals flowing in theopposite direction can be explained in the same way as mentioned later.

The lower-layer apparatus 310 comprises lower-layer communication-linesignal-receiver parts 330 and 331, lower-layer communication-linefailure-detecting parts 360 and 361, a lower-layer communication-linebridge/selector part 430, a lower-layer communication-linemultiplexing/demultiplexing part 450, a lower-layer communication-lineswitching-decision part 710, an AIS-transmission decision part 750 forsending an AIS to the upper layer, signal-transceiver parts 340-343 forsending out signals through the upper-layer communication lines, and AIStransmission parts 470-473 for sending out an AIS to the upper layer.The upper-layer apparatus 113 comprises upper-layer communication-linesignal-receiver parts 133 and 134, upper-layer communication-linefailure-detecting parts 163 and 164, an upper-layer communication-linebridge/selector part,230, and an upper-layer communication-lineswitching-decision part 720.

Now it is assumed that failure occurred in the lower-layer communicationline 600. The lower-layer communication-line failure-detecting part 360detects the failure and sends a failure signal to the lower-layercommunication-line switching-decision part 710 and the AIS-transmissiondecision part 750. Failure is detected by using an error-detectingsystem of LOS (Loss of Signal) or BIP (Byte Interleaved Parity) in thecase of SONET or SDH. CRC (Cyclic Redundancy Check) is used in case ofATM. AIS is transmitted by using the format defined by ANSI T1.105 orITU-T G.707 in case of SONET or SDH. OAM cells defined by ITU-T I.610are used in the case of ATM.

The AIS-transmission decision part 750 instructs the AIS transmissionparts 470-473 to send out an AIS through all the upper-layercommunication lines 500-503. The lower-layer communication-lineswitching-decision part 710 exchanges the switching protocol with itsopposite lower-layer apparatus to switch the communication line to theprotection lower-layer communication line 601. Although the switchingsystem shown in FIG. 6 is of a 1+1 type, switching protocol of the 1+1type, the 1:N type, and BLSR (Bidirectional Line Switched Ring) definedby ANSI T1.105 are available. The system will be described based on a1+1 type for the sake of simplification. In a system of a 1+1 type, thereceiving apparatuses, which have detected failure in the workingcommunication line, have their bridge/selector part switch thecommunication line to the protection communication line. Namely, basedon the information from the lower-layer communication-linefailure-detecting parts 360 and 361, the lower-layer communication-lineswitching-decision part 710 instructs the lower-layer communication-linebridge/selector part 430 to choose normal one between the working andprotection lower-layer communication lines 600 and 601. When theswitching in the lower layer has made correctly, the lower-layercommunication-line bridge/selector part 430 informs the AIS-transmissiondecision part 750 that switching in the upper layer is unnecessary,namely, that the AIS transmission to the upper-layer apparatuses is nolonger necessary. When the AIS-transmission decision part 750 determinesthat the switching has been made correctly by the lower-layerapparatuses and the transmission of the AIS, is no longer necessary, itinstructs the AIS transmission parts 470-473 to stop sending the alarmto the upper layer. Besides, the AIS-transmission decision part 750receives the failure information from both the lower-layercommunication-line failure-detecting parts 360 and 361 and determineswhether the line chosen by the lower-layer communication-linebridge/selector part 430 is correct or not. If there is inconsistencybetween the failure information on the two communication lines and theprocessing of the bridge/selector part, the AIS-transmission decisionpart 750 instructs the AIS transmission parts 470-473 to continuesending the AIS to the upper layer.

The upper-layer apparatuses 110-113 receive the AIS through theupper-layer communication lines 500-503 and carry out independently theswitching processing of the upper-layer communication lines inaccordance with the switching protocol of the upper layer.

Taking the upper-layer apparatus 113 as an example, the switchingprocessing by the upper layer apparatuses will be described. Theupper-layer communication-line failure-detecting parts 163 and 164detect the AIS coming through the upper-layer communication line 503,determine that the data transmitted through the upper-layercommunication line 503 are invalid, and inform the upper-layercommunication-line switching-decision part 720. The upper-layercommunication-line switching-decision part 720 exchanges the switchingprotocol with the opposite upper-layer apparatus to switch thecommunication line to the protection upper-layer communication line 504.Several types of switching protocol are available. For ATM, switchingprotocol of a 1+1 type is defined by the ITU-T Draft New RecommendationI.630.

If the upper-layer communication-line failure-detecting parts 163 and164 detect failure in the upper-layer communication line 503 instead ofan AIS coming through the upper-layer communication line 503, theupper-layer communication-line switching-decision part 720 exchanges theswitching protocol with the opposite apparatus to switch thecommunication line to the protection upper-layer communication line 504.

Although omitted in FIG. 6, the upper-layer apparatus 113 may have asignal-transceiver part for processing and sending signals to a furtherupper layer, and an AIS-transmission decision part and an AIStransmission part for transmitting an AIS to the further upper layer.

Referring to FIG. 9, the first embodiment of switching device includinga coordinated-switching decision part of the present invention will bedescribed in detail. FIG. 9 shows the details of a part of the networkof FIG. 2 in case that the network is constructed of the above lower-and upper-layer apparatuses.

The lower and upper layers in the embodiments of the present inventionare applicable to any of ATM, SONET, SDH, and OTN which constitute ahierarchic correlation shown in Table 1. They are also applicable to anyof the virtual path, the virtual channel, the path, and the line ofsubnetworks of ATM, SONET, and SDH. The idea, or design, in theembodiments of the present invention is applicable to any system whichhas a hierarchic correlation in accordance with the layer hierarchy ofthe OSI (Open System Interconnection) reference model defined by ISO.

The lower-layer apparatus 310 of FIG. 9 is connected with anotherlower-layer apparatus through the working and protection lower-layercommunication lines 600 and 601. The switching system of FIG. 9 is of a1+1 type having a set of a working line and a protection line, throughwhich the same data are transmitted while the conditions of thecommunication lines are normal. However, the embodiments of the presentinvention are applicable to a system having switching protocol of a 1:Ntype, the system having a number (“N”) of working lines and a singleprotection line. Besides, although a linear-type network is shown inFIG. 2, the idea, or design, in the embodiments of the present inventionis similarly applicable to ring-type networks and mesh-type networks.Description of a network with the protocol of a 1+1 linear type willfollow.

The lower-layer apparatus 310 is also connected with the upper-layerapparatuses 110-113 through the upper-layer communication lines 500-503.The upper-layer apparatus 113 has a protection upper-layer communicationline 504, which connects the upper-layer apparatus 113 with anotherlower-of upper-layer apparatus. The same data as the upper-layercommunication line 503 can be transmitted through the protectionupper-layer communication line 504 as the occasion demands.

Now, the system will be described, assuming that signals are flowingfrom the left-hand side to the right-hand side in FIG. 9. The systemwith signals flowing in the opposite direction can be explained in thesame way as mentioned later.

As in the case of the lower-layer apparatus 310 of FIG. 6, thelower-layer apparatus 310 of FIG. 9 comprises lower-layercommunication-line signal-receiver parts 330 and 331, lower-layercommunication-line failure-detecting parts 360 and 361, a lower-layercommunication-line bridge/selector part 430, a lower-layercommunication-line multiplexing/demultiplexing part 450, a lower-layercommunication-line switching-decision part 710, an AIS-transmissiondecision part 750 for determining whether or not to send an AIS to theupper layer, signal transceiver parts 340-343 for sending out signalsthrough the upper-layer communication lines 500-503, and AIStransmission parts 470-473 for sending out an AIS to the upper layer.However, the lower-layer communication-line switching-decision part 710and the AIS-transmission decision part 750 are not shown in FIG. 9. Thelower-layer apparatus 310 further comprises upper-layercommunication-line failure-detecting parts 460-463 for the workinglower-layer communication line 600, upper-layer communication-linefailure-detecting parts 465-468 for the protection lower-layercommunication line 601, and switching-inhibit-signal transmission parts480-483 for sending out switching-inhibit signals to the upper layer.These additional features are not included in conventional systems.

The switching-inhibit signal may be transmitted through opticalsupervisory channel by using the different wavelength from the signalsin the case of OTN, by using the overhead in the case of SONET or SDH,and by using the OAM (Operation Administration Maintenance) cell in thecase of ATM, etc.

As in the case of conventional apparatuses, the upper-layer apparatus113 of FIG. 9 comprises upper-layer communication-line signal-receiverparts 133 and 134, upper-layer communication-line failure-detectingparts 163 and 164, an upper-layer communication-line bridge/selectorpart 230, and an upper-layer communication-line switching-decision part720. However, the upper-layer communication-line switching-decision part720 is not shown in FIG. 9.

As in the case of the upper-layer apparatus 113, the upper-layerapparatuses 110-112 have upper-layer communication-line signal-receiverparts 130-132 and upper-layer communication-line failure-detecting parts160-162.

The upper-layer apparatus 113 further comprises abypass-information-signal transmission part 803, which conventionalsystems do not have. In the same way, the upper-layer apparatuses110-112 have bypass-information-signal transmission parts 800-802.

The bypass-information signal may be transmitted by using thewavelength-multiplexed supervisory signal in the case of OTN, by usingthe overhead in the case of SONET or SDH, and by using the OAM(Operation Administration Maintenance) cell, etc. in the case of ATM.

The bypass means a usable upper-layer communication line which goesthrough a lower-layer apparatus other than the lower-layer apparatus310. Whether there is a bypass or not for each upper-layer communicationline can be determined based upon the information on the network givenby the operator or the information on line failure from the upper-layercommunication-line failure-detecting part. One simple method ofdetermining whether each upper-layer communication line has a bypass ornot is as follows. When the operator designs a network, the operatorgrasps whether each upper-layer communication line has a bypass or notand input the bypass information into the bypass-information-signaltransmission part so that it can transmit the bypass information to thelower-layer apparatus. Another simple method is as follows. Whether eachupper-layer communication line has a bypass or not is determined on thesingle basis of whether the protection upper-layer communication line isnormal or failure. Namely, when the upper-layer communication-linefailure-detecting part 164 has detected failure in the protectionupper-layer communication line 504, the bypass-information-signaltransmission part 803 determines that a bypass is present, andaccordingly sends out a signal to the lower-layer apparatus. When theupper-layer communication-line failure-detecting part 164 has notdetected failure in the protection upper-layer communication line 504,the bypass-information-signal transmission part 803 determines that abypass is absent, and accordingly sends out a signal to the lower-layerapparatus. Furthermore, the logical sum (OR) of the bypass informationinputted by the operator and the information on whether each protectionupper-layer communication line is normal or failure may be found. Inthis case, the bypass-information-signal transmission parts 800-802 sendout bypass-available signals only when the operator's informationindicates the presence of a bypass and the protection upper-layercommunication line is normal.

Provided in this embodiment of the present invention is aninterlayer-coordinated-switching decision part 700. In FIG. 9, theinterlayer-coordinated-switching decision part 700 is disposed outsidethe lower-layer apparatus 310 and the upper-layer apparatuses 110-113.However, the interlayer-coordinated-switching decision part 700 may bedisposed inside the lower-layer apparatus 310 or any of the upper-layerapparatuses 110-113.

The interlayer-coordinated-switching decision part 700 receives thefailure information on the upper-layer communication lines from theupper-layer communication-line failure-detecting parts 160-164 of theupper-layer apparatuses 110-113, the failure information on thelower-layer communication lines from the lower-layer communication-linefailure-detecting parts 360 and 361 of the lower-layer apparatus 310,the failure information on the upper-layer communication lines from theupper-layer communication-line failure-detecting parts 460-463 and465-468 of the lower-layer apparatus 310.

The interlayer-coordinated-switching decision part 700 has the functionto check the failure information on the lower-layer communication lineswith that on the upper-layer communication lines when it has receivedfailure information from the failure-detecting parts. Theinterlayer-coordinated-switching decision part 700 controls thelower-layer communication-line bridge/selector part 430 and theswitching-inhibit-signal transmission parts 480-483 of the lower-layerapparatus 310. Besides, the interlayer-coordinated-switching decisionpart 700 receives bypass information from the bypass-information-signaltransmission parts 800-803 of the upper-layer apparatuses 100-103.

FIG. 17 shows the detailed configuration of theinterlayer-coordinated-switching decision part 700. Theinterlayer-coordinated-switching decision part 700 comprises afailure-information collecting part 900 for collecting the failureinformation from the lower-layer apparatus, a failure-informationcollecting part 920 for collecting the failure information from theupper-layer apparatuses, a failure-information checking part 910 forchecking the failure information from the lower-layer apparatus withthat from the upper-layer apparatuses, a state-calculating part 930, astate-choosing part 940, and a switching-inhibit-signal demanding part950 for demanding the lower-layer apparatus to send out aswitching-inhibit signal to the upper layer.

The failure-information collecting part 900 for collecting the failureinformation from the lower-layer apparatus has a table 901 fordescribing the failure information on the working lower-layercommunication line 600 sent from the lower-layer apparatus and a table902 for describing the failure information on the protection lower-layercommunication line 601 sent from the lower-layer apparatus. Thefailure-information collecting part 920 for collecting the failureinformation from the upper-layer apparatuses has a table 921 fordescribing the failure information on the working upper-layercommunication lines sent from the upper-layer apparatuses. Thefailure-information checking part 910 for checking the failureinformation from the lower-layer apparatus with that from theupper-layer apparatuses has a table 911 for describing the informationon effective working upper-layer communication lines and a table 912 fordescribing the information on the effective protection upper-layercommunication lines.

The state-calculating part 930 has a table 931 for calculating the firststate, a table 932 for calculating the second state, a table 933 forcalculating the third state, and a table 934 for calculating the fourthstate. The state-choosing part 940 has a decision of the state with themaximum number of available communication lines part 941 and a decisionof the state with the minimum number of protection switching part 942.The failure-information collecting part 900 is connected with the demandof the protection switching inhibit signal part 950 through aninterrupt-signal line 960. The part 950 is connected with theswitching-inhibit-signal transmission parts 480-483 and instructs themwhether or not to send out a switching-inhibit signal in accordance withthe decision of the interlayer-coordinated-switching decision part 700.

The table 901 for describing the failure information on the workinglower-layer communication line 600 receives the failure information fromthe working lower-layer communication-line failure-detecting part 360and the failure information from the upper-layer communication-linefailure-detecting parts 460-463 corresponding to the working lower-layercommunication line 600 and describes the failure information for eachupper-layer communication line. The table 902 for describing the failureinformation on the protection lower-layer communication line 601receives the failure information from the protection lower-layercommunication-line failure-detecting part 361 and the failureinformation from the upper-layer communication-line failure-detectingparts 465-468 corresponding to the protection lower-layer communicationline 601 and describes the failure information for each upper-layercommunication line.

The table 921 for describing the failure information on the workingupper-layer communication lines receives the failure information fromthe upper-layer communication-line failure-detecting parts 160-163corresponding to the working upper-layer communication lines 500-503 anddescribes the failure information for each upper-layer communicationline.

The table 911 for describing the information on effective workingupper-layer communication lines describes the information on each ofnondefective ones of the working upper-layer communication lines500-503, a nondefective line meaning a line with no failure between thelower-layer apparatus 310 and the upper-layer apparatus 110, 111, 112,or 113, as the case may be. Effective working upper-layer communicationlines are identified by logically subtracting the contents of the table901 of failure information on the working lower-layer communication linefrom the contents of the table 921 of failure information on the workingupper-layer communication lines. Namely, in case that the upper-layercommunication line 500 is described as defective on the table 921 andthe same line is described as normal on the table 901, it can beconsidered that a failure occurred in the upper-layer communication line500 between the lower-layer apparatus 310 and the upper-layer apparatus110. Accordingly, the upper-layer communication line 500 is described asunusable in the table 911 of information on effective workingupper-layer communication lines. In case that the upper-layercommunication line 500 is described as defective on the table 921 andthe same line is described again as defective on the table 901, it canbe considered that failure occurred on the left side of the lower-layerapparatus 310 in FIG. 9, namely, in the lower-layer communication line600 or above the line. Accordingly, the upper-layer communication line500 is described as usable in the table 911. If the failure-detectingmethod of the lower-layer apparatus and that of the upper-layerapparatuses are not identical with each other, and if the upper-layercommunication line 500 is described as normal in the table 921 and thesame line is described as defective in the table 901, it is consideredthat the upper-layer apparatus is receiving data normally and thedescription in the table 921 is adopted. Accordingly, the upper-layercommunication line 500 is described as usable in the table 911.

The table 912 for describing the information on effective protectioncommunication lines describes the information from thebypass-information-signal transmission parts 800-803 for eachupper-layer communication line. In other words, the table 912 describesa protection line for each of the working upper-layer communicationlines 500-503 so that when one of the upper-layer apparatuses 110-113has detected a failure in its working line, the communication line canbe shifted to a protection line.

There is not strict rule for dividing cases into states and the numberof tables to be prepared. Now, an embodiment with four state tables willbe described. In the first state, only the working communication linesare used in both the lower and upper layers. In the second state, theworking communication line is used in the lower layer and nondefectiveone of the working and protection communication lines is used by eachupper-layer apparatus. In the third state, the protection communicationline is used in the lower layer and the working communication lines areused in the upper layer. In the fourth state, the protectioncommunication line is used in the lower layer and nondefective one ofthe working and protection communication lines is used by eachupper-layer apparatus.

Now, the values of each state table will be described. Each state tabledescribes which communications lines can be chosen when coordinatedswitching has been completed in accordance with the state.

In the first state, because no switching takes place in both the lowerand upper layers, the state table, or state-calculating table, 931 hasthe same values as the table 921 of failure information on the workingupper-layer communication lines.

In the second state, because no switching takes place in the lower layerbut switching takes place in the upper layer, the values of the table921 of failure information on working upper-layer communication linesand the values of the table 912 of information on effective protectionupper-layer communication lines are compared path by path and the bettervalues are adopted by the state-calculating table 932 (if a path has anondefective line, its value is adopted).

In the third state, because switching takes place in the lower layer butno switching takes place in the upper layer, the logical sum (OR) of thevalue of the table 902 of failure information on the protectionlower-layer communication line and the value of the table 911 ofinformation on effective working upper-layer communication lines isfound path by path and adopted by the state-calculating table 933.

In the fourth state, switching takes place in both the lower and upperlayers, the value of the third-state calculating table 933 and the valueof the table 912 of effective protection upper-layer communication linesare compared path by path and the better value is adopted by thestate-calculating table 934 (if a path has a nondefective line, itsvalue is adopted).

The numbers of usable upper-layer communication lines and the numbers oftimes of switching are calculated from the first- to fourth-statecalculating tables 931-934, and the state-choosing part 940 chooses thebest state. The number of usable upper-layer communication lines in eachstate is the number of normal lines described as usable in thecalculating table of said state, 931, 932, 933, or 934, as the case maybe. Based on the numbers of usable upper-layer communication lines inthe four states, the decision of the state with the maximum number ofavailable communication lines part 941 extracts the state which has thelargest number of available communication lines. If two or more stateshave one and the same largest number, the part 941 extracts thosestates. Then, the part 941 passes the extracted state or states to thepart 942 for extracting the state with the smallest number of times ofprotection switching.

The number of times of protection switching is calculated as follows. Inthe first state, because no switching takes place in both the lower andupper layers, the number of times of protection switching is zero. Inthe third state, because switching takes place in the lower layer alone,the number of times of protection switching is one. In the second state,because switching takes place in the upper layer alone, the upper-layercommunication lines described as defective in the table 901 of failureinformation on the working lower-layer communication line are switched,if this state is extracted. Therefore, the number of times of protectionswitching is equal to the number of the lines described as defective inthe table 901. In the fourth state, switching takes place in both thelower and upper layers. In the upper layer, the upper-layercommunication lines described as defective in the table 902 of failureinformation on the protection lower-layer communication line areswitched, if this state is extracted. Therefore, the number of times ofprotection switching in the upper layer is equal to the number of thelines described as defective in the table 902. The number in the upperlay and the number of times of switching in the lower layer, one, add upto the total number of times of protection switching.

Based on the above calculation results, the part 942 for extracting thestate with the smallest number of times of protection switching extractsa state which has the minimum number of times of protection switching.If two or more states have one and the same largest number of availablecommunication lines and one and the same smallest number of times ofprotection switching, a state requiring no switching in the lower layeris chosen because switching in a single layer (the upper layer in thiscase) requires a shorter time than switching in both the layers. Namely,if two or more states have one and the same largest number of availablecommunication lines and one and the same smallest number of times ofprotection switching, the part 942 chooses not the third or fourthstate, but the second state.

When the part 942 has chosen a state requiring switching in the lowerlayer, namely, the third or fourth state, the lower-layercommunication-line bridge/selector part 430 of the lower-layer apparatus310 is activated to choose the protection lower-layer communication line601. When the switching in the lower layer has been completed, or incase that switching in the lower layer is unnecessary (in other words,the first or second state has been chosen), the switching-inhibit-signaldemanding part 950 is notified that it is no longer necessary tocontinue sending out the switching-inhibit signal to the upper layer.Then, the switching-inhibit-signal demanding part 950 instructs theswitching-inhibit-signal transmission parts 480-483 to stop sending outthe switching-inhibit signal to the upper layer. Then, switchingprocessing begins in the upper layer.

The processing procedure for coordinated switching in case of theoccurrence of failure will be described below.

The upper-layer communication-line failure-detecting parts 460-463 and465-468 of the lower-layer apparatus 310 in this embodiment are capableof detecting failure in the upper-layer communication lines line byline.

When the lower-layer communication-line failure-detecting part 360 orthe upper-layer communication-line failure-detecting parts 460-463 havedetected failure, they send the failure information to theinterlayer-coordinated-switching decision part 700 and the AIStransmission parts 470-473. Before or when the AIS transmission parts470-473 send out an AIS to the upper-layer apparatuses 110-113, theinterlayer-coordinated-switching decision part 700 instructs theswitching-inhibit-signal transmission parts 480-483 to send out aswitching-inhibit signal to the upper-layer apparatuses 110-113 so thatswitching does not begin in the upper layer.

In the above process, when the interlayer-coordinated-switching decisionpart 700 has received a failure signal from any of the lower-layercommunication-line failure-detecting part 360 and the upper-layercommunication-line failure-detecting parts 460-463, thefailure-information collecting part 900 for collecting the failureinformation from the lower-layer apparatus generates and sends out aninterrupt signal to the part 950 through-the interrupt-signal line 960.Upon the receipt of the interrupt signal, the part 950 instructs theswitching-inhibit-signal transmission parts 480-483 to send out aswitching-inhibit signal to the upper layer.

Then, the interlayer-coordinated-switching decision part 700 collectsfailure information for a prescribed time period from the upper-layercommunication-line failure-detecting parts 460-463 and 465-468 of thelower-layer apparatus 310 and describes the failure information for eachupper-layer communication line in the table 901 of failure informationon the working lower-layer communication line and in the table 902 offailure information on the protection lower-layer communication line. Itis to cope with probable time lag in transmitting failure informationfrom the plurality of failure-detecting parts to theinterlayer-coordinated-switching decision part 700. Such time lag mayoccur, regarding one and the same failure, due to different transmissionspeeds, different transmission paths, etc.

Besides, the interlayer-coordinated-switching decision part 700 collectsfailure information for the prescribed time period mentioned above fromthe upper-layer communication-line failure-detecting parts 160-163 andthe bypass-information-signal transmission parts 800-803 of theupper-layer apparatuses 110-113. Then, by using the checking methoddescribed earlier in the paragraphs of the details of theinterlayer-coordinated-switching decision part 700, theinterlayer-coordinated-switching decision part 700 checks the failureinformation from the lower-layer apparatus 310 with that from theupper-layer apparatuses 110-113.

Each of FIGS. 18, 19, and 20 shows an example of the processing of theinterlayer-coordinated-switching decision part 700. In the example ofFIG. 18, the upper-layer communication-line failure-detecting parts460-462 corresponding to the working lower-layer communication line 600detected failure, and the upper-layer communication-linefailure-detecting part 468 corresponding to the protection lower-layercommunication line 601 detected failure. Besides, the upper-layercommunication-line failure-detecting parts 160-162 of the upper-layerapparatuses 110-112 detected failure of the upper-layer communicationlines 500-502, and the bypass-information-signal transmission part 801of the upper-layer apparatus 111 indicates that there is no bypass,meaning that the protection line of the upper-layer communication line501 is not usable.

The nonfailure and failure information from the lower- and upper-layerapparatuses is described as “0” and “1”, respectively, in thefailure-information tables 901, 902, and 921. The information on theavailability of bypasses in the upper layer sent from the upper-layerapparatuses is described as “0” standing for a usable line and “1”standing for an unusable line in the table 912 of information oneffective protection upper-layer communication lines.

Described in the table 911 of information on effective workingupper-layer communication lines is the remainder after subtracting thecontents of the table 901 from the contents of the table 921. It can beconsidered in this example that the failure occurred above thelower-layer apparatus 310 (on the left side in FIG. 9). Therefore, theupper-layer communication lines 500-503 between the lower-layerapparatus 310 and the upper-layer apparatuses 110-113 are essentiallynormal and usable.

Using the contents of these tables 901, 902, 911, 912, and 921,calculation is carried out for the four states. In the first statewherein no switching takes place in both the lower and upper layers, thestate-calculating table 931 has the same values as thefailure-information table 921.

In the second state wherein no switching takes place in the lower layerbut switching takes place in the upper layer, the values of thefailure-information table 921 and the values of the information table912 are compared path by path and the better, or smaller, as shown inthe example of FIG. 18, value is adopted by the state-calculating table932.

In the third state wherein switching takes place in the lower layer butno switching takes place in the upper layer, the logical sum (OR) of thevalue of the failure-information table 902 and the value of theinformation table 911 is found path by path and adopted by thestate-calculating table 933.

In the fourth state wherein switching takes place in both the upper andlower layers, the value of the third-state calculating table 933 and thevalue of the information table 912 are compared path by path and thebetter, or smaller, value is adopted by the state-calculating table 934.

The number of usable upper-layer communication lines and the number oftimes of switching in each state are calculated from thestate-calculating table of said state 931, 932, 933, or 934, as the casemay be. The number of usable upper-layer communication lines in eachstate is the number of normal lines described as usable in thestate-calculating table of said state 931, 932, 933, or 934, as the casemay be. In this example, the fourth state wherein switching takes placein both the upper and lower layers has the largest number of usablecommunication lines, four, among the four states.

Because the fourth state alone was chosen by the part 941 for extractingthe state with the largest number of available communication lines, thefourth state is adopted by the part 942 for extracting the state withthe smallest number of times of protection switching.

Then, the lower-layer communication-line bridge/selector part 430 of thelower-layer apparatus 310 is activated to choose the protectionlower-layer communication line 601. When switching in the lower layerhas been completed, the part 950 is notified that it is no longernecessary to continue sending out the switching-inhibit signal to theupper-layer apparatuses. Then, the part 950 instructs theswitching-inhibit-signal transmission parts 480-483 to stop sending outthe switching-inhibit signal to the upper-layer apparatuses. Then,switching processing begins in the upper layer.

Now, the example of FIG. 19 will be described. In this example, theupper-layer communication-line failure-detecting part 462 correspondingto the working lower-layer communication line 600 detected failure, andthe upper-layer communication-line failure-detecting part 468corresponding to the protection lower-layer communication line 601detected failure. Besides, the upper-layer communication-linefailure-detecting parts 160 and 161 detected failure of the upper-layercommunication lines 500 and 502, respectively, and everybypass-information-signal transmission part 800-803 has indicated thatthere is a bypass, meaning that all the protection communication linesof the upper layer are usable.

As in the case of the first example, the above nonfailure and failureinformation from the lower- and upper-layer apparatuses is described inthe failure-information tables 901, 902, and 921. The information on theavailability of bypasses in the upper layer sent from the upper-layerapparatuses is described in the information table 912.

Described in the information table 911 is the remainder aftersubtracting the contents of the failure-information table 901 from thecontents of the failure-information table 921. In this example, thefailure occurred in the upper-layer communication line 500 between thelower-layer apparatus 310 and the upper-layer apparatus 110. If thelower-layer apparatus 310 makes line switching to restore the failedline, the upper-layer communication line 500 remains unusable.

As in the case of the first example, calculation is carried out for thefour states to obtain the state-calculating table 931-934. Then, thenumber of usable upper-layer communication lines and the number of timesof switching in each state are calculated from the state-calculatingtable of said state 931, 932, 933, or 934, as the case may be. In thisexample, the second and fourth states have one and the same largestnumber of usable communication lines, that is, four. Accordingly, thepart 941 chooses the second and fourth states and notifies the result tothe part 942.

The numbers of times of protection switching in this example arecalculated as follows. The number of times of protection switching inthe second state is the number of communication lines described asdefective in the failure-information table 901, that is, one. The numberof times of protection switching in the fourth state is the sum of thenumber of communication lines described as defective in thefailure-information table 902 and the number of times of switching inthe lower layer, that is, one, the sum being two. Accordingly, the part942 for extracting the state with the smallest number of times ofprotection switching chooses the second state.

In the second state, switching takes place in the upper layer while noswitching takes place in the lower layer. Accordingly, the part 942notifies the part 950 that it is no longer necessary to continue sendingout the switching-inhibit signal to the upper-layer apparatuses. Then,the part 950 instructs the switching-inhibit-signal transmission parts480-483 to stop sending out the switching-inhibit signal to theupper-layer apparatuses. Then, switching processing begins in the upperlayer.

Now, the example of FIG. 20 will be described. In this example, theupper-layer communication-line failure-detecting parts 460 and 462corresponding to the working lower-layer communication line 600 havedetected failure, and the upper-layer communication-linefailure-detecting part 468 corresponding to the protection lower-layercommunication line 601 has detected failure. Besides, the upper-layercommunication-line failure-detecting parts 160, 161, and 162 havedetected failure of the upper-layer communication lines 500, 501, and502, respectively, and every bypass-information-signal transmission part801 has indicated that there is a bypass, meaning that all theprotection communication lines in the upper layer are usable.

As in the case of the first example, the above nonfailure and failureinformation from the lower- and upper-layer apparatuses is described inthe failure-information tables 901, 902, and 921. The information on theavailability of bypasses in the upper layer sent from the upper-layerapparatuses is described in the information table 912.

Described in the information table 911 is the remainder aftersubtracting the contents of the failure-information table 901 from thecontents of the failure-information table 921. In this example, thefailure occurred in the upper-layer communication line 501 between thelower-layer apparatus 310 and the upper-layer apparatus 111. If thelower-layer apparatus 310 makes line switching to restore the failedlines, the upper-layer communication line 501 remains unusable.

As in the case of the first example, calculation is carried out for thefour states to obtain the state-calculating table 931-934. Then, thenumber of usable upper-layer communication lines and the number of timesof switching in each state are calculated from the state-calculatingtable of said state 931, 932, 933, or 934, as the case may be. In thisexample, the second and fourth states have one and the same largestnumber of usable communication lines, that is, four. Accordingly, thepart 941 chooses the second and fourth states and notifies the result tothe decision of the state with the minimum number of protectionswitching part 942.

In either state, the number of times of protection switching is found tobe two. Accordingly, the two states remain chosen at the part 942. Iftwo or more states have one and the same largest number of usablecommunication lines and one and the same smallest number of times ofprotection switching, a state requiring no switching in the lower layeris chosen because switching in a single layer requires a shorter timethan switching in both the layers. In accordance with this choosingrule, the part 942 chooses the second state. Then, the part 942instructs the demand of the protection switching inhibit signal part950, which in turn instructs the switching-inhibit-signal transmissionparts 480-483 to stop sending out the switching-inhibit signal to theupper-layer apparatuses. Then, switching processing begins in the upperlayer.

It is not necessary for the upper-layer communication-linefailure-detecting parts 460-463 and 465-468 of the lower-layer apparatus310 to be identical with the upper-layer communication-linefailure-detecting parts 160-164 of the upper-layer apparatuses 110-113.The failure-detecting system of the failure-detecting parts 460-463 and465-468 may be simplified or may make bundles of lines to detect failurebundle by bundle. In this case, however, checking the failureinformation from the lower-layer apparatus with that from theupper-layer apparatuses results in inconsistency, and even when failurehas occurred above the lower-layer apparatus 310, the failure isregarded as existing between the lower-layer apparatus 310 andupper-layer apparatuses 110-113.

When lower-layer apparatus 310 has detected failure, but the upper-layerapparatuses 110-113 have not detected the failure, the detecting resultof the upper-layer apparatuses 110-113 is given priority, theupper-layer communication lines regarded as functioning normally.

FIG. 16 is a flowchart of the processing of theinterlayer-coordinated-switching decision part 700 to realize the firstembodiment of the present invention.

Although described above is a network which has the switching protocolof the 1+1 type and has a set of a working line and a protection onethrough which the same data are transmitted during the normal operationof the network, the embodiment of the present invention is applicable toa network having the protocol of the 1:N type.

In the case of the switching protocol of the 1:N type, no data aretransmitted through the protection communication line or the data of oneon the “N” communication lines are transmitted through the protectionline under the condition of no auxiliary line while the network isoperating normally. When switching processing has begun, the systemshifts into a state called “bridge” wherein the data of the failedworking communication line is switched to the protection line. Then, thesystem shifts into a state called “switch” wherein the receiving sidechooses the protection line to complete the switching processing. Thestate of the protection line can be determined in the bridge state.Namely, in case that the embodiment of the present invention is appliedto a network with the protocol of the 1:N type, when theinterlayer-coordinated-switching decision part 700 has detected failurein the working lower-layer communication line 600, the part 700instructs the switching-inhibit-signal transmission parts 480-483 tosend out a switching-inhibit signal to the upper layer and thelower-layer communication-line bridge/selector part 430 to shift intothe bridge state. Accordingly, the interlayer-coordinated-switchingdecision part 700 can determine the state of the protection lower-layercommunication line 601. The switching processing thereafter can becarried out as in the case of the protocol of the 1+1 type. In addition,because the embodiment of the present invention does not depend on aring-type network, a mesh-type network, or the like, it is applicable tosuch networks. The first embodiment can be reduced to an embodimentcomprising a simpler set of apparatuses by omitting some functions fromthe first embodiment.

Second Embodiment

The second embodiment of switching device with a coordinated-switchingdecision part of the present invention will now be described. FIG. 10 isa conceptual illustration of the embodiment. The second embodiment isthe first embodiment less the function-of conveying the information onwhether the upper-layer apparatuses have bypasses or not from the upperlayer to the coordinated-switching decision part.

FIG. 11 shows the configuration of the second embodiment of switchingdevice of the present invention in detail. FIG. 11 shows the details ofa part of the network of FIG. 2 in case that the network is constructedof conventional lower- and upper-layer apparatuses.

The second embodiment is the first embodiment less the function ofconveying the information on bypasses in the upper layer. Theinterlayer-coordinated-switching decision part 700 uses the informationon whether protection lines in the upper layer are normal or failure,instead of the bypass information. Because the switching processing inthis embodiment may be carried out in the same way as the switchingprocessing in the first embodiment, its description is omitted here.

Third Embodiment

The third embodiment of switching device with a coordinated-switchingdecision part of the present invention will now be described. FIG. 12 isa conceptual illustration of the embodiment. The third embodiment is thesecond embodiment less the function of conveying upper-layercommunication-line failure information from the upper-layer apparatusesto the interlayer-coordinated-switching decision part.

FIG. 13 shows the configuration of the third embodiment of switchingdevice of the present invention in detail. FIG. 13 shows the details ofa part of the network of FIG. 2 in case that the network is constructedof conventional lower- and upper-layer apparatuses.

The interlayer-coordinated-switching decision part 700 does not checkthe failure information from the lower-layer apparatus 310 with thatform the upper-layer apparatuses 100-103, but controls the switching inthe lower layer and instructs the switching-inhibit-signal transmissionparts 480-483 to stop sending out the switching-inhibit signal to theupper layer, on the basis of the failure information from thelower-layer apparatus 310.

The switching-control method of the third embodiment differs from thatof the first embodiment. In the third embodiment, theinterlayer-coordinated-switching decision part 700 does not receivefailure information from the upper-layer apparatuses. Therefore, tosecure the maximum number of communication lines based on the availablefailure information is to choose one with better quality between theworking communication line 600 and the protection communication line601. In this embodiment, the number of times of switching inclusive ofswitching in the upper layer can not be minimized because theinterlayer-coordinated-switching decision part 700 dose not receivefailure information from the upper-layer apparatuses.

Another method of switching the communication line between the workingand protection lines in the lower layer is to prioritize the upper-layercommunication lines and switch the communication line according to thepriority. For example, in FIG. 18, there is no bypass for thecommunication line 2 in the upper layer. If the operator of thelower-layer apparatus knows it, the operator gives top priority to thecommunication line 2, that is to say, the lower-layer apparatus switchesthe communication line in the lower layer to either the working or theprotection line which can communicate with the line 2. In the case ofthe example of FIG. 18, this switching method results in the sameswitching processing in both the lower and upper layers as the firstembodiment wherein bypass information is used. The operator may useother criteria than the availability of bypasses in prioritizing theupper-layer communication lines. Once the switching-control method isdecided, the interlayer-coordinated-switching decision part 700 makesline switching in the lower layer, lifts the switching-inhibit signal tothe upper layer apparatuses, and makes line switching in the upperlayer, as in the case of the first embodiment.

Fourth Embodiment

The fourth embodiment of switching device with a coordinated-switchingdecision part of the present invention will now be described. Theconceptual illustration of the second embodiment shown in FIG. 10 isapplicable to this embodiment. This embodiment is the second embodimentless the upper-layer communication-line failure-detecting parts of thelower-layer apparatus 310.

FIG. 14 shows the configuration of this embodiment of switching deviceof the present invention in detail. FIG. 14 shows the details of a partof the network of FIG. 2 in case that the network is constructed ofconventional lower- and upper-layer apparatuses.

This embodiment is effective in case that the distance between thelower-layer apparatus and the upper-layer apparatuses is short; forexample, they are disposed in the same-central office, or the componentsor communication lines connecting the lower-layer apparatus and theupper-layer apparatuses are highly reliable, and hence the probabilityof occurrence of failure in the section between the lower-layerapparatus and the upper-layer apparatuses is significantly lower thanthose of the other sections.

The interlayer-coordinated-switching decision part 700 controls lineswitching in the lower layer and lifts the switching-inhibit signal tothe upper-layer apparatuses 110-113 based on the failure informationsent from the upper-layer communication-line failure-detecting parts160-164 of the upper-layer apparatuses 110-113. Theinterlayer-coordinated-switching decision part 700 controls lineswitching so as to secure the largest number of normal communicationlines through which the upper-layer apparatuses can receive data. As inthe case of the other embodiments, while line switching is taking placein the lower layer, a switching-inhibit signal is sent out to theupper-layer apparatuses.

As described above, the present invention provides a switching systemwhich comprises two layers, each having an independent switchingfunction, and utilizes both the switching functions, taking fulladvantage of the features of the functions in accordance with theconditions of the communication lines, without disenabling eitherfunction.

In other words, by utilizing both the switching functions in accordancewith the line conditions, the largest number of communication lines canbe made available for the upper-layer apparatuses. Besides, theswitching can be controlled so as to minimize the number of times ofswitching. In the case of switching in both the layers in particular,because switching takes place in the lower layer first and then in theupper layer, the switching is made securely, with less exchange of theprotocol and high reliability. Besides, the total switching time in casethat switching takes place in both the layers can be significantlyreduced. FIG. 15 is a time chart of an embodiment of the presentinvention wherein the upper layer is an ATM network, its hold-off timebeing 500 msec, and the lower layer is a SONET, its switching-processingtime being 50 msec. Although the switching in the two layers takes about550 msec in case of the conventional method shown in FIG. 7, theswitching in the two layers takes only 100 msec or so in case of theembodiment of the present invention.

Now, the arts in relation to the embodiments of the present inventionwill be recited below.

1. A communication apparatus which:

is connected with a network comprising

(i) lower-layer apparatuses which are connected with communication linesof a lower layer and communication lines of an upper layer and havemeans for line switching in the lower layer, and

(ii) upper-layer apparatuses which are connected with the communicationlines of the upper layer and have means for line switching in the upperlayer; and

detects line failure and coordinates line switching by theline-switching means of the lower- and upper-layer apparatuses by usingfailure information on the lower- and upper-layer communication lines.

2. A communication apparatus according to the above paragraph 1 whereineach lower-layer apparatus has a coordinated-switching device or acoordinated-switching function or is connected withcoordinated-switching means through an interface.

3. A communication apparatus according to the above paragraph 1 having aswitching mode that the line-switching means of the upper-layerapparatuses begin switching when the line switching means of the of thelower-layer apparatuses have finished switching.

4. A communication apparatus which:

is connected with a network comprising

(i) lower-layer apparatuses which are connected with communication linesof a lower layer and communication lines of an upper layer and havemeans for line switching in the lower layer, and

(ii) upper-layer apparatuses which are connected with the communicationlines of the upper layer and have means for line switching in the upperlayer; and

has means for

(i) detecting line failure and determining the order of switching by theswitching means of the lower- and upper-layer apparatuses by usingfailure information on the lower- and upper-layer communication lines,and

(ii) restoring faulty communication lines.

5. A communication apparatus according to the above paragraph 4 whereinsaid means is the lower-layer apparatuses each of which has acoordinated-switching device or a coordinated-switching function or isconnected with coordinated-switching means through an interface.

6. A communication apparatus which:

is connected with a network comprising

(i) lower-layer apparatuses which are connected with communication linesof a lower layer and communication lines of an upper layer and havemeans for line switching in the lower layer, and

(ii) upper-layer apparatuses which are connected with the communicationlines of the upper layer and have means for line switching in the upperlayer;

detects line failure and collects failure information on the lower- andupper-layer communication lines;

identifies one or more faulty lines of the lower or the upper layer andlocates the site of occurrence of failure by using the collected failureinformation;

identifies, by using the information on the faulty communication linesand the site of occurrence of failure, lower- and upper-layerapparatuses which are required to make line switching in order torestore the faulty communication lines; and

has means for determining the order and the timing of line switching bythe line-switching means of the identified lower- and upper-layerapparatuses.

7. A communication apparatus according to the above paragraph 6 whereinsaid means is the lower-layer apparatuses each of which has acoordinated-switching device or a coordinated-switching function or isconnected with coordinated-switching means through an interface.

8. A communication apparatus which:

is connected with a network comprising

(i) lower-layer apparatuses which are connected with communication linesof a lower layer and communication lines of an upper layer and havemeans for line switching in the lower layer, and

(ii) upper-layer apparatuses which are connected with the communicationlines of the upper layer and have means for line switching in the upperlayer;

transmits a switching-inhibit signal to the upper-layer apparatuses whenany lower-layer apparatus has detected line failure;

identifies faulty lines of the lower and upper layers and locates thesite of occurrence of failure by using failure information collected onthe lower- and upper-layer communication lines; and

coordinates line switching by the lower- and upper-layer apparatuses byusing said collected failure information on the lower- and upper-layercommunication lines.

9. A communication apparatus according to the above paragraph 8 wherein,in case that the lower-layer apparatuses have detected line failure, aswitching-inhibit signal is transmitted to the upper-layer apparatusesfirst and then the failure detection is notified to them.

10. A communication apparatus according to the above paragraph 8 whereineach lower-layer apparatus has a coordinated-switching device or acoordinated-switching function or is connected withcoordinated-switching means through an interface.

11. A communication apparatus which:

is connected with a network comprising

(i) lower-layer apparatuses which are connected with communication linesof a lower layer and communication lines of an upper layer and havemeans for line switching in the lower layer, and

(ii) upper-layer apparatuses which are connected with the communicationlines of the upper layer and have means for line switching in the upperlayer;

transmits a switching-inhibit signal to the upper-layer apparatuses whenany lower-layer apparatus has detected line failure;

identifies one or more faulty lines of the lower or the upper layer andlocates the site of occurrence of failure by using failure informationcollected on the lower- and upper-layer communication lines;

cancels the switching-inhibit signal to the upper-layer apparatuses whenno lower-layer communication line has-been found faulty but any of theupper-layer communication lines has been found faulty;

determines, by using failure information on the lower- and upper-layercommunication lines, which line-switching means of the lower- andupper-layer apparatuses should make switching in order to secure alargest number of normal upper-layer communication lines, or in order torestore high-priority lines rather than low-priority lines, or in orderto secure a largest number of signal channels, in case that any of thelower-layer communication lines has been found faulty; and

has means for causing line switching in the lower layer first and thencanceling the switching-inhibit signal to the upper-layer apparatuses incase that switching is to take place in both the lower and upper layersin accordance with said determination.

12. A communication apparatus according to the above paragraph 11wherein, in case that the lower-layer apparatuses have detected linefailure, a switching-inhibit signal is transmitted to the upper-layerapparatuses first and then the failure detection is notified to them.

13. A communication apparatus according to the above paragraph 11wherein each lower-layer apparatus has a coordinated-switching device ora coordinated-switching function or is connected withcoordinated-switching means through an interface.

14. A communication system wherein:

provided are a first and a second upper-layer apparatus and a first, asecond, a third, and a fourth lower-layer apparatus, each apparatushaving line-switching means;

(i) the first and second upper-layer apparatuses are connected to eachother through a first and a second upper-layer communication linebetween which communication can be switched, (ii) the first and secondlower-layer apparatuses are connected to each other through a first anda second lower-layer communication line between which communication canbe switched, each lower-layer communication line being multiplexed toaccommodate one or more upper-layer communication lines, (iii) the thirdand fourth lower-layer apparatuses are connected to each other through athird and a fourth lower-layer communication line between whichcommunication can be switched, each lower-layer communication line beingmultiplexed to accommodate one or more upper-layer communication lines,(iv) the first upper-layer communication line connects the firstupper-layer apparatus with the first lower-layer apparatus and thesecond upper-layer apparatus with the second lower-layer apparatus, and(v) the second upper-layer communication line connects the firstupper-layer apparatus with the third lower-layer apparatus and thesecond upper-layer apparatus with the fourth lower-layer apparatus;

a switching-inhibit signal is transmitted to the upper-layer apparatuseswhen the lower-layer apparatuses have detected line failure;

one or more faulty lines of the lower or the upper layer are identifiedand the site of occurrence of failure is located by using failureinformation on the lower- and upper-layer communication lines;

the switching-inhibit signal to the upper-layer apparatuses is cancelledwhen no lower-layer communication line has been found faulty but any ofthe upper-layer communication lines has been found faulty;

which line-switching means of the lower- and upper-layer apparatusesshould make switching is determined, based on failure information on thelower- and upper-layer communication lines, in order to secure a largestnumber of normal upper-layer communication lines, or in order to restorehigh-priority lines rather than low-priority lines, or in order tosecure a largest number of signal channels, in case that any of thelower-layer communication lines has been found faulty; and

provided is a first means for causing line switching in the lower layerfirst and then canceling the switching-inhibit signal to the upper-layerapparatuses, in case that switching is to take place in both the lowerand upper layers in accordance with said determination.

15. A communication system according to the above paragraph 14 whereinsaid first means:

chooses a set of the line-switching means which has the smallest numberof times of switching if two or more sets of line-switching means arefound to bring about one and the same result for the proposition ofsecuring a largest number of normal upper-layer communication lines,restoring high-priority lines rather than low-priority lines, orsecuring a largest number of signal channels;

chooses a set of line-switching means requiring no line switching in thelower layer, if any, and the switching-inhibit signal to the upper-layerapparatuses is cancelled if two or more sets of line-switching means arefound to have one and the same smallest number of times of switching;and

causes line switching in the lower layer first and then cancels theswitching-inhibit signal to the upper-layer apparatuses if there is nota set of line-switching means which requires no line switching in thelower layer.

16. A communication system according to the above paragraph 14 wherein,in case that the lower-layer apparatuses have detected line failure, aswitching-inhibit signal is transmitted to the upper-layer apparatusesfirst, and then the failure detection is notified to them.

17. A communication system wherein:

a plurality of lower-layer apparatuses and a plurality of upper-layerapparatuses are provided, the latter belonging to a layer which is upperthan a layer to which the former belong, each apparatus havingline-switching means;

connected between the lower-layer apparatuses are lower-layercommunication lines, each lower-layer communication line beingmultiplexed to accommodate one or more upper-layer communication lines,and connected between the upper-layer apparatuses are upper-layercommunication lines going through the lower-layer apparatuses;

a switching-inhibit signal is transmitted to the upper-layer apparatuseswhen any of the lower-layer apparatuses has detected line failure;

one or more faulty lines of the lower or the upper layer are identifiedand the site of occurrence of failure is located based on failureinformation on the lower- and upper-layer communication lines;

the switching-inhibit signal to the upper-layer apparatuses is cancelledwhen no lower-layer communication line has been found faulty but any ofthe upper-layer communication lines has been found faulty;

which line-switching means of the lower- and upper-layer apparatusesshould make switching is determined, based on failure information on thelower- and upper-layer communication lines, in order to secure a largestnumber of normal upper-layer communication lines, or in order to restorehigh-priority lines rather than low-priority lines, or in order tosecure a largest number of signal channels, in case that any of thelower-layer communication lines has been found faulty; and

provided is a first means for causing line switching in the lower layerfirst and then canceling the switching-inhibit signal to the upper-layerapparatuses, in case that switching is to take place in both the lowerand upper layers in accordance with said determination.

18. A communication system according to the above paragraph 17 wherein,in case that no lower-layer communication line has been found faulty butany of the upper-layer communication lines has been found faulty, theswitching-inhibit signal to the upper-layer apparatuses is cancelledfirst and then the failure detection is notified to them.

19. A communication system wherein:

provided are a first and a second upper-layer apparatus and a first, asecond, a third, and a fourth lower-layer apparatus, each apparatushaving line-switching means;

(i) the first and second upper-layer apparatuses are connected to eachother through a first and a second upper-layer communication linebetween which communication can be switched, (ii) the first and secondlower-layer apparatuses are connected to each other through a first anda second lower-layer communication line between which communication canbe switched, each lower-layer communication line being multiplexed toaccommodate one or more upper-layer communication lines, (iii) the thirdand fourth lower-layer apparatuses are connected to each other through athird and a fourth lower-layer communication line between whichcommunication can be switched, each lower-layer communication line beingmultiplexed to accommodate one or more upper-layer communication lines,(iv) the first upper-layer communication line connects the firstupper-layer apparatus with the first lower-layer apparatus and thesecond upper-layer apparatus with the second lower-layer apparatus, and(v) the second upper-layer communication line connects the firstupper-layer apparatus with the third lower-layer apparatus and thesecond upper-layer apparatus with the fourth lower-layer apparatus;

a switching-inhibit signal is transmitted to the upper-layer apparatuseswhen the lower-layer apparatuses have detected line failure;

faulty lines of the lower and upper layers are identified and the siteof occurrence of failure is located based on failure information on thelower- and upper-layer communication lines;

the switching-inhibit signal to the upper-layer apparatuses is cancelledwhen no lower-layer communication line has been found faulty but any ofthe upper-layer communication lines has been found faulty; and

provided is a first means for determining, based on failure informationon the lower- and upper-layer communication lines, which line-switchingmeans of the lower- and upper-layer apparatuses should make switching inorder to secure a largest number of normal upper-layer communicationlines, or in order to restore high-priority lines rather thanlow-priority lines, or in order to secure a largest number of signalchannels, in case that any of the lower-layer communication lines hasbeen found faulty.

20. A communication system according to the above paragraph 19 wherein,in case that switching is to take place in both the lower and upperlayers in accordance with said determination, line switching is made inthe lower layer first and then the switching-inhibit signal to theupper-layer apparatuses is cancelled.

21. A communication system according to the above paragraph 19 whereinsaid first means:

chooses a set of the line-switching means which has the smallest numberof times of switching if two or more sets of line-switching means arefound to bring about one and the same result for the proposition ofsecuring a largest number of normal upper-layer communication lines,restoring high-priority lines rather than low-priority lines, orsecuring a largest number of signal channels;

chooses a set of line-switching means requiring no line switching in thelower layer, if any, and the switching-inhibit signal to the upper-layerapparatuses is cancelled if two or more sets of line-switching means arefound to have one and the same smallest number of times of switching;and

causes line switching in the lower layer first and then cancels theswitching-inhibit signal to the upper-layer apparatuses if there is nota set of line-switching means which requires no line switching in thelower layer.

22. A communication system according to the above paragraph 19 wherein,in case that the lower-layer apparatuses have detected line failure, aswitching-inhibit signal is transmitted to the upper-layer apparatusesfirst and then the failure detection is notified to them.

23. A communication system wherein:

a plurality of lower-layer apparatuses and a plurality of upper-layerapparatuses are provided, the latter belonging to a layer which is upperthan a layer to which the former belong, each apparatus havingline-switching means;

connected between the lower-layer apparatuses are lower-layercommunication lines, each lower-layer communication line beingmultiplexed to accommodate one or more upper-layer communication lines,and connected between the upper-layer apparatuses are upper-layercommunication lines going through the lower-layer apparatuses;

a switching-inhibit signal is transmitted to the upper-layer apparatuseswhen any of the lower-layer apparatuses has detected line failure;

one or more faulty lines of the lower or the upper layer are identifiedand the site of occurrence of failure is located based on failureinformation on the lower- and upper-layer communication lines;

the switching-inhibit signal to the upper-layer apparatuses is cancelledwhen no lower-layer communication line has been found faulty but any ofthe upper-layer communication lines has been found faulty; and

provided is a first means for determining, based on failure informationon the lower- and upper-layer communication lines, which line-switchingmeans of the lower- and upper-layer apparatuses should make switching inorder to secure a largest number of normal upper-layer communicationlines, or in order to restore high-priority lines rather thanlow-priority lines, or in order to secure a largest number of signalchannels, in case that any of the lower-layer communication lines hasbeen found faulty.

24. A communication system according to the above paragraph 23 wherein,in case that switching is to take place in both the lower and upperlayers in accordance with said determination, line switching is made inthe lower layer first and then the switching-inhibit signal to theupper-layer apparatuses is cancelled.

25. A communication system according to the above paragraph 23 wherein,in case that no lower-layer communication line has been found faulty butany of the upper-layer communication lines has been found faulty, theswitching-inhibit signal to the upper-layer apparatuses is cancelledfirst and then the failure detection is notified to them.

26. A communication apparatus which:

is connected with a network comprising

(i) lower-layer apparatuses which are connected with communication linesof a lower layer and communication lines of an upper layer and havemeans for line switching in the lower layer, and

(ii) upper-layer apparatuses which are connected with the communicationlines of the upper layer and have means for line switching in the upperlayer; and

has means for giving an instruction to relevant upper-layer apparatusesin case that line failure can not partly or wholly be made good by lineswitching of relevant lower-layer apparatuses, the instruction requiringsaid relevant upper-layer apparatuses to make line switching to bypassesgoing around said relevant lower-layer apparatuses.

27. A communication apparatus according to the above paragraph 26wherein each lower-layer apparatus has a coordinated-switching device ora coordinated-switching function or is connected withcoordinated-switching means through an interface.

28. A communication apparatus which:

is connected with a network comprising

(i) lower-layer apparatuses which are connected with communication linesof a lower layer and communication lines of an upper layer and havemeans for line switching in the lower layer, and

(ii) upper-layer apparatuses which are connected with the communicationlines of the upper layer and have means for line switching in the upperlayer;

detects line failure and finds one or more faulty lines of the upper- orthe lower-layer communication lines and the site of occurrence offailure based on failure information on the upper- and lower-layercommunication lines;

determines which upper- and lower-layer apparatuses should make lineswitching based on said information on faulty lines and the site ofoccurrence of failure and information on whether or not there arebypasses going around faulty lower-layer apparatuses, if any; and

has means for giving an instruction to the relevant upper-layerapparatuses, in case that line failure can not partly or wholly be madegood by line switching of relevant lower-layer apparatuses and there arebypasses going round said relevant lower-layer apparatuses, theinstruction requiring said relevant upper-layer apparatuses to make lineswitching to said bypasses.

29. A communication apparatus according to the above paragraph 28wherein each lower-layer apparatus has a coordinated-switching device ora coordinated-switching function or is connected withcoordinated-switching means through an interface.

30. A communication apparatus according to the above paragraph 28wherein said failure information on the upper-layer communication linesis collected from at least one of the upper-layer apparatus group andthe lower-layer apparatus group.

31. A communication apparatus according to the above paragraph 28wherein said instruction is to cancel the switching-inhibit signal tothe upper-layer apparatuses.

32. A communication apparatus which:

is connected with a network comprising

(i) lower-layer apparatuses which are connected with communication linesof a lower layer and communication lines of an upper layer and havemeans for line switching in the lower layer, and

(ii) upper-layer apparatuses which are connected with the communicationlines of the upper layer and have means for line switching in the upperlayer;

detects line failure and, at the same time, finds one or more faultylines of the upper- or the lower-layer communication lines and the siteof occurrence of failure by using information on whether bypasses goingaround particular lower-layer apparatuses can be secured or not by lineswitching of relevant upper-layer apparatuses and failure information onthe upper- and lower-layer communication lines;

determines which upper- and lower-layer apparatuses should make lineswitching based on said information on faulty lines and the site ofoccurrence of failure; and

has means for instructing the upper- and lower-layer apparatuses to makeline switching in the lower layer first and then make line switching inthe upper layer in case that line switching is required in both theupper and lower layers.

33. A communication apparatus according to the above paragraph 32wherein each lower-layer apparatus has a coordinated-switching device ora coordinated-switching function or is connected withcoordinated-switching means through an interface.

34. A communication system comprising:

(i) lower-layer apparatuses which are connected with communication linesof a lower layer and communication lines of an upper layer and havemeans for line switching in the lower layer and (ii) upper-layerapparatuses which are connected with the communication lines of theupper layer and have means for line switching in the upper layer; and

means for giving an instruction to relevant upper-layer apparatuses incase that line failure can not partly or wholly be made good by lineswitching of relevant lower-layer apparatuses, the instruction requiringsaid relevant upper-layer apparatuses to make line switching to bypassesgoing around said relevant lower-layer apparatuses.

35. A communication system according to the above paragraph 34 whereineach lower-layer apparatus has a coordinated-switching device or acoordinated-switching function or is connected withcoordinated-switching means through an interface.

36. A communication system which:

has (i) lower-layer apparatuses which are connected with communicationlines of a lower layer and communication lines of an upper layer andhave means for line switching in the lower layer, and (ii) upper-layerapparatuses which are connected with the communication lines of theupper layer and have means for line switching in the upper layer;

detects line failure and finds one or more faulty lines of the upper- orthe lower-layer communication lines and the site of occurrence offailure based on failure information on the upper- and lower-layercommunication lines;

determines which upper- and lower-layer apparatuses should make lineswitching based on said information on faulty lines and the site ofoccurrence of failure and information on whether or not there arebypasses going around faulty lower-layer apparatuses, if any; and

has means for giving an instruction to relevant upper-layer apparatuses,in case that line failure can not partly or wholly be made good by lineswitching of relevant lower-layer apparatuses and there are bypassesgoing round said relevant lower-layer apparatuses, the instructionrequiring said relevant upper-layer apparatuses to make line switchingto said bypasses.

37. A communication system according to the above paragraph 36 whereinsaid failure information on the upper-layer communication lines iscollected from at least one of the upper-layer apparatus group and thelower-layer apparatus group.

38. A communication system according to the above paragraph 36 whereinsaid instruction is to cancel the switching-inhibit signal to theupper-layer apparatuses.

39. A communication system which:

has (i) lower-layer apparatuses which are connected with communicationlines of a lower layer and communication lines of an upper layer andhave means for line switching in the lower layer, and (ii) upper-layerapparatuses which are connected with the communication lines of theupper layer and have means for line switching in the upper layer;

detects line failure and, at the same time, finds one or more faultylines of the upper- or the lower-layer communication lines and the siteof occurrence of failure by using information on whether bypasses goingaround particular lower-layer apparatuses can be secured or not by lineswitching of relevant upper-layer apparatuses and failure information onthe upper- and lower-layer communication lines;

determines which upper- and lower-layer apparatuses should make lineswitching based on said information on faulty lines and the site ofoccurrence of failure; and

has means for instructing the upper- and lower-layer apparatuses to makeline switching in the lower layer first and then make line switching inthe upper layer in case that line switching is required in both theupper and lower layers.

In accordance with the present invention, line switching coordinatedbetween the two layers can be made.

What is claimed is:
 1. A communication apparatus which is connected witha network comprising: (i) lower-layer apparatuses which are connectedwith communication lines of a lower layer and communication lines of anupper layer, each lower-layer apparatus having means for line switchingin the lower layer; (ii) upper-layer apparatuses which are connectedwith the communication lines of the upper layer and have means for lineswitching in the upper layer, wherein said lower-layer apparatustransmits a switching-inhibit signal to the upper-layer apparatuses whensaid lower-layer apparatus has detected a line failure, in a lower orupper layer communication line identifies one or more faulty lines ofthe lower or the upper layer and locates the site of occurrence offailure by using failure information collected on the lower andupper-layer communication lines, cancels the switching-inhibit signal tothe upper-layer apparatuses when no lower-layer communication line hasbeen found faulty but an upper-layer communication line has been foundfaulty,; determines, by using failure information concerning the lowerand upper-layer communication lines, which line-switching means of thelower and upper-layer apparatuses should perform switching in order tosecure a largest number of normal upper-layer communication lines, or inorder to restore high-priority lines rather than low-priority lines, orin order to secure a largest number of signal channels, in case that anyof the lower-layer communication lines has been found faulty; and meansfor causing line switching in the lower layer first and then cancelingthe switching-inhibit signal to the upper-layer apparatuses whenswitching is to take place in both the lower and upper layers inaccordance with said determination.
 2. A communication systemcomprising: first and second upper-layer apparatuses first, second,third, and fourth lower-layer apparatus, each apparatus having lineswitching means, (i) wherein the first and second upper-layerapparatuses are connected to each other through first and secondupper-layer communication lines between which communication can beswitched, (ii) the first and second lower-layer apparatuses areconnected to each other through first and second lower-layercommunication lines between which communication can be switched, eachlower-layer communication line being multiplexed to accommodate one ormore upper-layer communication lines, (iii) the third and fourthlower-layer apparatuses are connected to each other through third andfourth lower-layer communication line between which communication can beswitched, each lower-layer communication line being multiplexed toaccommodate one or more upper-layer communication lines, (iv) the firstupper-layer communication line connects the first upper-layer apparatuswith the first lower-layer apparatus and the second upper-layerapparatus with the second lower-layer apparatus, and (v) the secondupper-layer communication line connects the first upper-layer apparatuswith the third lower-layer apparatus and the second upper-layerapparatus with the fourth lower-layer apparatus, wherein aswitching-inhibit signal is transmitted to the upper-layer apparatuseswhen the lower-layer apparatuses have detected a line failure, on saidlower and upper layer communication lines, one or more faulty lines ofthe lower or the upper layer are identified and the site of occurrenceof failure is located by using failure information on the lower andupper-layer communication lines, the switching-inhibit signal to theupper-layer apparatuses is cancelled when no lower-layer communicationline has been found faulty but as upper-layer communication line hasbeen found faulty, whether line-switching means of the lower andupper-layer apparatuses should perform switching is determined, based onfailure information on the lower and upper-layer communication lines, inorder to secure a largest number of normal upper-layer communicationlines, or in order to restore high-priority lines rather thanlow-priority lines, or in order to secure a largest number of signalchannels, in case that any of the lower-layer communication lines hasbeen found faulty; and first means for causing line switching in thelower layer first and then canceling the switching-inhibit signal to theupper-layer apparatuses, when switching is to take place in both thelower and upper layers in accordance with said determination.
 3. Acommunication system according to claim 2 wherein said first means:chooses a set of the line-switching means which has the smallest numberof times of switching if two or more sets of line-switching means arefound to bring about one and the same result for the proposition ofsecuring a largest number of normal upper-layer communication lines,restoring high-priority lines rather than low-priority lines, orsecuring a largest number of signal channels, chooses a set ofline-switching means requiring no line switching in the lower layer, ifany, and the switching-inhibit signal to the upper-layer apparatuses iscancelled if two or more sets of line-switching means are found to haveone and the same smallest number of times of switching, and causes lineswitching in the lower layer first and then cancels theswitching-inhibit signal to the upper-layer apparatuses if there is nota set of line-switching means which requires no line switching in thelower layer.
 4. A communication system comprising: a plurality oflower-layer apparatuses; a plurality of upper-layer apparatuses whichbelong to a layer which is upper than a layer to which the lower-layerapparatuses belong, wherein each apparatus has line-switching means,wherein connected between the lower-layer apparatuses are lower-layercommunication lines, each lower-layer communication line beingmultiplexed to accommodate one or more upper-layer communication lines,and connected between the upper-layer apparatuses are upper-layercommunication lines going through the lower-layer apparatuses, aswitching-inhibit signal is transmitted to the upper-layer apparatuseswhen any of the lower-layer apparatuses has detected line failure, onsaid lower and upper layer communication lines, one or more faulty linesof the lower or the upper layer are identified and the site ofoccurrence of failure is located based on failure information on thelower and upper-layer communication lines, the switching-inhibit signalto the upper-layer apparatuses is cancelled when no lower-layercommunication line has been found faulty but an upper-layercommunication line has been found faulty, whether line-switching meansof the lower and upper-layer apparatuses should perform switching isdetermined, based on failure information on the lower and upper-layercommunication lines, in order to secure a largest number of normalupper-layer communication lines, or in order to restore high-prioritylines rather than low-priority lines, or in order to secure a largestnumber of signal channels, any of the lower-layer communication lineshas been found faulty; and first means for causing line switching in thelower layer first and then canceling the switching-inhibit signal to theupper-layer apparatuses, when switching is to take place in both thelower and upper layers in accordance with said determination.
 5. Acommunication system comprising: first and second upper-layerapparatuses; first, second, third, and fourth lower-layer apparatuses,wherein each apparatus has line-switching means, (i) wherein the firstand second upper-layer apparatuses are connected to each other throughfirst and second upper-layer communication line between whichcommunication can be switched, (ii) the first and second lower-layerapparatuses are connected to each other through first and secondlower-layer communication lines between which communication can beswitched, each lower-layer communication line being multiplexed toaccommodate one or more upper-layer communication lines, (iii) the thirdand fourth lower-layer apparatuses are connected to each other throughthird and fourth lower-layer communication lines between whichcommunication can be switched, each lower-layer communication line beingmultiplexed to accommodate one or more upper-layer communication lines,(iv) the first upper-layer communication line connects the firstupper-layer apparatus with the first lower-layer apparatus and thesecond upper-layer apparatus with the second lower-layer apparatus, and(v) the second upper-layer communication line connects the firstupper-layer apparatus with the third lower-layer apparatus and thesecond upper-layer apparatus with the fourth lower-layer apparatus,wherein a switching-inhibit signal is transmitted to the upper-layerapparatuses when the lower-layer apparatuses have detected a linefailure, on said lower and upper layer communication lines, faulty linesof the lower and upper layers are identified and the site of occurrenceof failure is located based on failure information on the lower- andupper-layer communication lines, and the switching-inhibit signal to theupper-layer apparatuses is cancelled when no lower-layer communicationline has been found faulty but upper-layer communication line has beenfound faulty; and first means for determining, based on failureinformation on the lower and upper-layer communication lines, whichline-switching means of the lower and upper-layer apparatuses shouldperform switching in order to secure a largest number of normalupper-layer communication lines, or in order to restore high-prioritylines rather than low-priority lines, or in order to secure a largestnumber of signal channels, in case that any-of the lower-layercommunication lines has been found faulty.
 6. A communication systemcomprising: a plurality of lower-layer apparatuses; a plurality ofupper-layer apparatuses which to a layer which is upper than a layer towhich the lower-layer apparatus belong, wherein each apparatus has lineswitching means, wherein connected between the lower-layer apparatusesare lower-layer communication lines, each lower-layer communication linebeing multiplexed to accommodate one or more upper-layer communicationlines, and connected between the upper-layer apparatuses are upper-layercommunication lines going through the lower-layer apparatuses, aswitching-inhibit signal is transmitted to the upper-layer apparatuseswhen any of the lower-layer apparatuses has detected a line failure, onsaid lower and upper layer communication lines, one or more faulty linesof the lower or the upper layer are identified and the site ofoccurrence of failure is located based on failure information on thelower and upper-layer communication lines, and the switching-inhibitsignal to the upper-layer apparatuses is cancelled when no lower-layercommunication line has been found faulty but any of the upper-layercommunication lines has been found faulty; and first means fordetermining, based on failure information on the lower and upper-layercommunication lines, which line-switching means of the lower- andupper-layer apparatuses should perform switching in order to secure alargest number of normal upper-layer communication lines, or in order torestore high-priority lines rather than low-priority lines, or in orderto secure a largest number of signal channels, in case that any of thelower-layer communication lines has been found faulty.